Work vehicles are configured to perform a wide variety of tasks for use as construction vehicles, forestry vehicles, lawn maintenance vehicles, as well as on-road vehicles such as those used to plow snow, spread salt, or vehicles with towing capability. Additionally, work vehicles include agricultural vehicles, such as a tractor or a self-propelled combine-harvester, which include a prime mover that generates power to perform work. In the case of a tractor, for instance, the prime mover is often a diesel engine that generates power from a supply of diesel fuel. The diesel engine drives a transmission which moves wheels or treads to propel the tractor across a field at a designated speed. Tractors often include a power takeoff (PTO) which includes a shaft coupled to the transmission and driven by the engine to power an implement being pulled or pushed through a field by the tractor.
Tractors can be steered through a field by a manual command provided by an operator through a manually controlled steering device such as a steering wheel or joystick, or by an automatic steering command. In the case of an automatic steering command, a steering control signal can be provided by a global positioning system (GPS) signal. Steering control systems often include one or more sensors configured to sense a position of the steering device or a position of the wheels with respect to a frame of the vehicle.
The speed of the tractor can also be controlled either manually or automatically as is known by those skilled in the art. Vehicle speed, when manually controlled, is adjusted through the use of an accelerator that is adjusted by a foot pedal or hand device. One type of automatic speed control is also known as “cruise control”.
In some tractors, the vehicle direction is also controlled by a vehicle direction control signal provided by the GPS signal to adjust the vehicle direction. Many agriculture tractors manufactured today include an on-board ability to automatically control the direction of the wheels of the tractor independently of the operator's steering wheel input. This capability paired with a position receiver and a guidance display automatically steers the tractor down a defined path in the field.
Such integrated guidance systems are configured to maintain the tractor and the implement, if included, on a fairly straight line path, without curving or deviating, by adjusting the direction of the steered wheels of the tractor only. These systems, however, struggle with maintaining direction along the defined path when the path's curvature approaches the tractor's minimum turning radius. At higher ground speeds, the tractor's turning radius gets larger. If the tractor's ground speed is not slowed as it encounters areas on the planned path with tight curvature segments, then the tractor tends to overdrive the path segment. This leads to a large path lateral tracking error and unacceptable guidance performance.
Generally the tractor needs to slow down to make a tight, space constrained, turn at the end of the field path. If the tractor does not slow sufficiently, the minimum radius the tractor can reliably guide itself will exceed the allotted space to actually make the turn. This forces the operator to manually slow the tractor down and then speed it up after the turn. Generally, the segments of the planned path with the tightest curvature segments tend to be around an end of row turn, but can also be encountered in the middle of a field with a curved path segment.
What is needed therefore is a speed control system that is configured to adjust ground speed of a vehicle based on the path being traveled.